A decompression valve in which a valve body is fitted around the outer periphery of a valve shaft with an O ring disposed between the valve body and the valve shaft, the O ring providing sealing between a valve chamber and a decompression chamber, and the O ring is disposed between the valve shaft and a guide member for guiding axial movement of the valve shaft, is known from Patent Document 1.    Patent Document 1: Japanese Patent Application Laid-open No. 2008-267558